Iron ore pelletizing process and product



United States Patent IRON ORE PELLETIZINGPROCESS AND PRODUCT Grover Justin Holt, Hibbing, Minm, and Louis .l. Erch, Negaunne, Mich., assignors to The Clcveland-Clifis Iron Company, Cleveland, Ohio, a corporation of Ohio No Drawing. Application April 3, 1953, Serial No. 346,786

5 Claims. (Cl. 75-5) The present invention relates to the ore dressing art and is more particularly concerned with novel pellets of iron ore and with a novel method of pelletizing finely divided iron ores.

The pelletizing of finely divided ores is a relatively old practiceand in some fields and in connection with certain ores has proven to be a commercially satisfactory means of preparing the ores for reduction. As'applied to iron ores and particularly magnetite, however, pelletizing has not heretofore proven commercially successful despite the great and increasingly urgent demand for an economical iron ore pelletizing process and the resulting numerous efforts by others to discover such a process. One of the most common diificulties has been the tendency for the'green pellets in firing to agglomerate and form clinkers. Theoretically, clinkers will not be formed where the firing or curing of the green pellets is carried out at a temperature just below the incipient fusion temperature of the pellet constituents. Thus, under ideal firing circumstances andwhen the pellets are composed of ore particles of proper size and distribution, a pellet product of the required physical characteristics will be consistently obtained in commercially acceptable yields. However, in practice it is extremely difiicult to maintain throughout a firing period the required control over the temperature to which. the pellets are subjected in the furnace, and in cases where exterior combustion chambers are employed the dilficulty of such control is even greater.

Prior attempts at solving the clinkering problem have not proven successful, one line of attack being to provide means-to break up clinkers formed in the furnace, it .apparently being assumed that this might be commercially feasible, while avoiding clinker formation would not be. .The clinkers can be reduced in this manner, but it is at a ruinous cost in terms of efficiency or yield. Also, such breaking up adds materially to the fines and dust which are not desired in blast furnace feed.

In accordance with our invention, finely divided iron ores consisting essentially of hematite or magnetite or combinations thereof may be pelletized in an eificient and economical manner with a minimum of waste and without formation of clinkers in significant amounts.

This invention is based on our discovery that by providing green iron ore pellets with a coating or layer of a certain type of material, they will be resistant to clinkering tendencies even thoughthe temperatures to which the pellets are subjected in firing range are above those normally employed or necessary adequately to cure the iron ore pellets.

Broadly, the method of the present invention comprisesthe steps of moistening iron ore fines, forming ball like bodies by rolling these moistened fines together, and rolling these bodies together with finely divided refractory material and formingon each body a shell of said refractory material which tenaciously adheres to and encases the iron ore body constituting the core of the resulting pellet.

green strength characteristics.

2,799,572 Patented July 16, 1957 The pellets made in acordance with this method have substantially no tendency to clinker in the burning or curing phase of their production. Furthermore, in accordance with the preferred practice of this invention these pellets in their green form uniquely combine valuable non-clinkering, non-dusting and high crushing strength characteristics and in their fired or cured form they have unusually high crushing strengths and therefore are ideally suited to the transportation and handling operations involved in delivering them from the point of their manufacture to the blast furnace, open hearth or other furnace in which they are to be charged. Still further, these pellets have an additional advantage over many of those produced in accordance with the prior art practices in that they are unusually porous and therefore quickly heated and readily reduced on contact with the hot carbon monoxide gases produced in the blast furnace.

In general, an iron ore pellet of this invention comprises a substantially spherical core composed of particles of iron ore, and a casing of substantially uniform thick; ness. covering said core and tenaciously adhering thereto, said casing being composed of one or more layers of refractory material having a melting point above about 2350 R, which prevents clinkering of these pellets when firing in the curing furnace.

The preliminary steps of this present method may suitably be carried out in conventional pelletizing apparatus consisting primarily of a revolving drum into which the ore fines are fed near one end from which the green pellets are discharged at the other end. Apparatus which may be used for this purpose is shown and described in detail in U. S. Patent No. 2,543,898, granted March 6, 1951, to Fred D. DeVaney.

As indicated above, the first step in the present method following introduction of the fines into a conventional pelletizing drum is to moisten or wet these fines. The amount of water employed in proportion to the amount of fines to be pelletized is regulated in accordance with this invention to produce a pellet having the requisite green strength to keep the pellet from breaking up or falling apart prior to the time that it is completely cured. We have found that when the proportion of moisture to fines is within the range of about 7% to about 17% adequate green strength will be assured. Hard, dense ore fines require less moisture than softer lighter fines for the same of bentonite in a given ore fines mixture will have a substantial bearing upon the amount of moisture required for a given adequate green strength, more moisture being needed where the bentonite content is high than when it is lower.

The formation of the balls or pellets of this invention is preferably accomplished by rolling the pelletizing drum on its axis so that the fines are rolled in contact with each other in such a way that substantially perfect spheroids are formed. The amount of fines and the length of the rolling period is regulated to produce pellets of the desired radii, although it will be understood that the pellets of a given batch of ore charged to the drum may be of different size within reasonable limits. Such limits will be determined by the economies of the pelletizing operation, the use to which the pellets are to be put and similar considerations, but these are matters collateral to this invention.

As the newly formed green pellets proceed toward the discharge end of the forming drum, they are rolled in contact with finely-divided, dry refractory material charged or provided in the drum at a point below the pellet-forming portion of the drum. However, in some operations it may be desirable to moisten the refractory material prior to rolling the green pellets in contact there- Likewise the proportion,

with and such variation in this procedure is contemplated by this invention and included in the appended claims. Due to the affinity of the moist pellets for the dry refractory material, a layer of refractory material is quickly formed on each pellet and is tenaciously held there by capillary action of the pellet moisture. Preferably, the application of the refractory material to the pellet is continued until in efiect several layers of refractory material have been formed thereon. In accordance with this invention, however, the foregoing results and advantages may be obtained where only a single layer of refractory material is applied to a pellet and this is so even where such a layer is in effect substantially only one particle thick. Those skilled in the art will realize that the important thing in this phase of the invention is the provision of a coat or layer on each pellet of such nature as to have substantially no tendency to bond adjacent pellets together at their points of contact under the temperature conditions prevailing in the shell portions of the pellets during the firing step. Thus, where the refractory material coat is composed in material amount of a substance which softens under furnace conditions because of a tendency to slag with one or more constituents of the core, or for any other reason, it is necessary to have a layer. or a plurality of layers of aggregate thickness substantially in excess of a single particle thickness if agglomeration or clinkering of the pellets is to be prevented. On the other hand, where a highly refractory material is employed in coating the green pellets and the constituents of the material do not tend to slag to material degree or to form solid solutions with the pellet core constituents to the extent that noticeable softening of the refractory coats on the pellets occurs to produce clinkering effects in firing,

only the thinnest type of refractory coating is required to produce the results and advantages of this invention.

A refractory material for use in accordance with this invention should have a melting point above about 2350 F., the maximum temperature to which iron ore pellets are normally subjected in firing at the present time. If higher temperatures or lower temperatures are to be used, the refractory material may be selected accordingly in order to provide the characteristics of temperature resistance essential to preventing clinkering tendencies during the firing operation. Ideally, the refractory material should have a crushing strength in excess of about 600 pounds per square inch and should be in the range of about mesh to about 28 mesh. When these conditions are met, coating of the green pellets with refractory material is easily and quickly accomplished and the crushing strength of the fired pellets will be consistently high. It will be understood, however, that consistently satisfactory results can be obtained through the use of this invention without both or either of these ideal conditions being established.

The refractory material may in the preferred embodiment of this invention contain relatively large quantities of slag and solid solution-forming constituents, it being understood, however, that a refractory material containing one or another of these constituents in sufiicient quantity may be used with essentially the same results being obtained. This ability to form slags or solid solutions may be in reference to any constituent of the pellets which is present in appreciable amount. Thus, this invention contemplates the use of iron oxides naturally present in such pellets and also contemplates the use of other naturally occurring pellet ingredients and any added materials. As those skilled in the art will understand, where slag or solid solution effects are obtained between the coating refractory material and the core material of the pellet, at least the base layer of refractory is bonded firmly and securely to the pellet, adding to the pellet strength and thus increasing the efficiency of the pelletizing process by minimizing or eliminating pellet breaka We have found that Allmul which is commercially available, is ideally suited as a refractory material for the coating of the pellets of iron ore in accordance with this invention. Allmul consists of 76.17% alumina, 20.08% silica and small to trace amounts of chromium, manganese and iron oxides. This material has a melting point of 3335 F. and has a cold (fired) crushing strength of approximately 3400 pounds per square inch.

When the desired thickness of refractory material has been deposited in a single layer or in multiple layers on the pellets and the pellets have reached the lower end of the drum, they are discharged from the drum, collected and subsequently charged into a suitable curing furnace where they are heated to a curing temperature for a length of time sufficient to produce in them the desired qualities set out above. On discharge from the curing furnace the pellets are finished and ready for shipment and use.

Having thus described the present invention so that others skilled in the arm may be able to gain a better understanding and practice the same, we state that What we desire to secure by Letters Patent is defined in what is claimed.

What is claimed is:

1. The method of making iron ore pellets which have substantially no tendency to clinker during burning or curing, which comprises the steps of moistening iron ore fines, forming ball-like bodies by rolling the moistened fines together, rolling these bodies in contact with comminuted refractory material having a melting point temperature above about 2350 F. and coating each of said bodies with a layer of said refractory material, and heating the resulting coated bodies while in contact with one another to a curing temperature below the melting point of said refractory material and thereby forming a solid solution composed of iron oxide of the ore fines and a metal oxide ingredient of the refractory material.

2. The method of making iron ore pellets which have substantially no tendency to clinker during burning or curing, which comprises the steps of moistening iron ore fines, forming ball-like bodies by rolling the moistened fines together, rolling these bodies in contact with cornminuted refractory material having a melting point temperature above about 2350 F. and providing a layer of said refractory material on said bodies, forming addi-v tional refractory material layers on said bodies, heating said bodies While in contact with one another to a curing temperature below the melting point-of. said refractory material and thereby forming on each body a shell of said refractory material tenaciously adhering to and encasing the said iron ore body constituting the core of the resulting pellets.

3. The method of making iron ore pellets which in green form uniquely combine valuable non-clinkering, non-dusting and .high crushing strength characteristics and which in burned or cured form have unusually high crushing strength, which comprises the steps of moistening iron ore fines, forming ball-like bodies by rolling the moistened fines together, rolling these bodies in contact with comminuted refractory material having a melting point above about 2350" F. and containing substantial amounts of slag-forming substances and coating each of said bodies with a layer of said refractory material, and curing the resulting coated bodies by heating them while in contact with one another to a temperature below the melting point of the refractory but sufiiciently high to form a solid solution coating layer composed of iron oxide and a metal oxide ingredient of the refractory material.

4. The method of making iron ore pellets which in green form uniquely combine valuable non-clinkering, non-dusting and high crushing strength characteristics and which in burned or cured form have unusually high crushing strength, which comprises the steps of moistenin giron ore fines, forming ball-like bodies by rolling the moistened fines together, and rolling these bodies in contact with comminuated refractory material having a melting point approximately about 3300 F. and composed of about 76% alumina and about 20% silica and providing a layer of said refractory material on said bodies, forming additional refractory material layers on said bodies, curing said bodies by heating them while in contact with one another to a temperature below the incipient fusion temperature of the refractory material but sufiiciently high to form on each body a relatively hard shell tenaciously adhering to and encasing the said iron ore body constituting the core of the resulting pellet.

5. An indurated iron ore pellet having unusually high crushing strength and comprising a core portion composed of particles of iron ore and a casing of refractory material covering the core and bonded thereto, said pellet having properties and characteristics obtainable by forming a ball-like body of moist iron ore fines, rolling this body in contact with comminuted refractory material having a melting point approximately 3300 F. and containing substantial amounts of slag-forming substances and coating said body with'a plurality of layers of said refractory material, and heating the thus covered body while in contact with other similar bodies to a temperature below the melting point of said refractory material but sufficiently high to form a slag bond between the refractory material and the core portion of the body.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES A. S. M. Review of Current Metal Literature, January 1949, page 23. 

2. THE METHOD OF MAKING IRON ORE PELLETS WHICH HAVE SUBSTANTIALLY NO TENDENCY TO CLINKER DURING BURNING OR CURING, WHICH COMPRISES THE STEPS OF MOISTENING IRON ORE FINES, FORMING BALL-LIKE BODIES BY ROLLING THE MOISTENED FINES TOGETHER, ROLLING THESE BODIES IN CONTACT WITH COMMINUTED REFRACTORY MATERIAL HAVING A MELTING POINT TEMPERATURE ABOVE ABOUT 2350* F. AND PROVIDING A LAYER OF SAID REFRACTORY MATERIAL ON SAID BODIES, FORMING ADDITIONAL REFRACTORY MATERIAL LAYERS ON SAID BODIES, HEATING SAID BODIES WHILE IN CONTACT WITH ONE ANOTHER TO A CURING TEMPERATURE BELOW THE MELTING POINT OF SAID REFRACTORY MATERIAL AND THEREBY FORMING ON EACH BODY A SHELL OF SAID REFRACTORY MATERIAL TENACIOUSLY ADHERING TO AND ENCASING THE SAID IRON ORE BODY CONSTITUTING THE CORE OF THE RESULTING PELLETS. 